86 
enemies. A number of the species inhabit shallow 
ponds, marshes, and ditches which are often dry 
during the summer, and they protect themselves 
by closing their shell with a filmy covering similar 
to that with which the land shells are closed during 
unfavourable weather ; the animal then retires into 
the interior of the shell, which, as a rule, is too large 
for its body, and there, in the moisture retained, 
awaits the return of the water. These mollusks are 
also furnished with organs enabling them to live by 
either air or water. The genus contains ten British 
species, of which only the following four are found 
in the ponds of the district. 
1. P. nitidus has a thin glossy light yellowish 
horn-coloured shell of from 4 to 5 whorls, and is one 
of the smallest of the genus. It has only as yet been 
found in Moncrieffe Pond, although it is said to be 
common throughout Britain. 
2. P. nautileus has a shell of 3 whorls, is much 
smaller than the last named species, and is easily 
distinguished from it and all'the other species by 
having the outside of the last whorl beautifuUy^ 
crested. It has been found in a pond at Old Scone, 
and in Moncrieffe pond. It is also said to be common 
throughout Britain, and will likely yet be found in 
many other places in the district. 
3. P. albus has a much larger shell than the pre¬ 
vious two, of a greyish white colour, and is found in 
the pond at the top of the North Inch, and in Scone 
and Moncreiffe ponds. 
4. P. contortus has a bright brown or reddish 
coloured shell formed of 8 whorls, very narrow and 
compressed. It is found abundantly in a pond near 
Pitlochry, in the pond at the top of the North Inch, 
and in Moncreiffe and Scone ponds. 
The second genus is Physa, in which we have only 
two species—the shells are spiral and have a remark¬ 
ably glossy appearance. 
1. P. fontinalis is found in Moncreiffe pond and 
many other localities in the district. It is very 
active in its habits, creeping and floating by jerks, 
which are supposed to be caused by the peculiar 
shape of its foot; it is said also to be able to spin a 
